Subclasses of external adenosine receptors.

Abstract

ABSTACT Cell surface adenosine receptors mediate either stimulation or inhibition of adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1], and the receptors that mediate these different responses can be discriminated with selected adenosine analogs. 5'-N-Ethylcarboxamideadenosine is a more potent agonist at stimulatory receptors (Ra) than is N8-phenylisopropyladenosine, whereas the reverse potency order is seen with inhibitory receptors (Ri). The potency of adenosine is intermediate between the potencies of these two analogs. The relative potencies of adenosine receptor agonists are maintained in physiological responses in intact cells, such as steroidogenesis and inhibition of lipolysis. As with adrenergic receptors, subelasses of adenosine receptors differ functionally and pharmacologically. Adenosine modifies the physiological function and cyclic AMP concentration in a large variety of cell types by interacting with external receptors (1-3), the basic properties of which were described by Sattin and Rall (4). In plasma membrane preparations from many different cell types, adenosine and several purine-modified analogs stimulate adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] (1-5), whereas in adipocyte membranes the adenosine receptor mediates decreased activity (1,3,6). Previously published studies suggested that, despite their superficial similarities, stimulatory and inhibitory adenosine receptors might differ. Thus, whereas N6-phenylisopropyladenosine (PIA) and N6-methyladenosine were equipotent in stimulating the Leydig tumor cell adenylate cyclase (5), the former analog was far more potent that N6-methyladenosine in inhibiting the adipocyte adenylate cyclase (7). However, the fat cell studies were performed in the presence of adenosine deaminase, added to metabolize adenosine resulting from breakdown of the substrate, ATP. In this case, the analogs might have acted either at the receptor or by inhibiting the adenosine deaminase, which has a rather broad substrate specificity (8). Another method to circumvent interference from "intrinsic" adenosine is the use of dATP as the cyclase substrate (9). Metabolism of this substrate yields 2'-deoxyadenosine, which has little activity at adenosine receptors. In this report we present a pharmacological investigation of stimulatory and inhibitory receptors associated with adenylate cyclases, using dATP as substrate in the absence of adenosine deaminase. From a screening of numerous adenosine analogs we have selected two that demonstrate the existence of subclasses of adenosine receptors: PIA and 5'-N-ethylcarboxamideadenosine (NECA). The relative potencies of the analogs in the adenylate cyclase studies are maintained in physiological studies in intact cells.* Fig. 1 presents a comparison of the concentration dependencies of adenosine, PIA, and NECA in their actions on three adenylate cyclase systems: liver and 1-10 Leydig cell enzymes, which are activated by adenosine, and rat adipocyte enzyme, which is inhibit...

Permissive stimulation of Ca(2+)-induced phospholipase A2 by an adenosine receptor agonist in a pertussis toxin-sensitive manner in FRTL-5 thyroid cells: a new ‘cross-talk’ mechanism in Ca2+ signalling.

“…All the adenosine derivatives tested enhanced Ca2+-induced release of arachidonic acid in the order of potency PIA = CHA > CADO = NECA. This rank order is in accordance with that of agonist potencies for the A1 receptor [23]. The results were practically the same as those obtained in the ionomycin-and thapsigargin-provoked events (results not shown).…”

Effects of Adenosine and its Analogues on Porcine Basilar Arteries: Are Only A2 Receptors Involved?

“…Londos et al (1980) showed that the A2 receptors in the liver had a very high affinity for NECA, an affinity much higher than those in the adrenal or Leydig cells. A possible third adenosine receptor has been identified in rat fat cells (Garcia-Sainz and To mer, 1985) and also centrally DeLo renzo, 1985, 1986).…”

Adenosine levels in the postimplantation mouse uterus: Quantitation by HPLC‐fluorometric detection and spatiotemporal regulation by 5′‐nucleotidase and adenosine deaminase

“…It is liberated from nucleotide pools under conditions of net ATP catabolism and, as a paracrine signal, invokes an array of physiological responses that collectively serve to protect cells during metabolic stress and tissue injury (Arch and Newsholme, 1978; for recent overviews, see Bruns, 1991;Engler and Gruber, 1991). Most of these actions are mediated by specific membrane adenosine receptors (A, and A,) which are coupled to signal transduction pathways by guanine nucleotide regulatory proteins (Londos et al, 1978(Londos et al, , 1980Maenhaut et al, 1990;Libert et al, 1991).…”